"The file login is a type of application/octet-stream and Netscape does not know how to handle this file"
The file had no extension? Just
login?
I got something similar by renaming a
binary file (I used a z ZIP file) so that it had no extension, then tried to open it within Netscape using File > Open File. I tried renaming a PDF file with no extension but Netscape recognized that filetype and used my acrobat reader plugin. Netscape was able to recognize other renamed files that it handles internally (I tried renaming a PNG and TXT file with no extension and Netscape opened those without asking).
I used a zip file renamed "TESTz" (no extension). If I tried to open the same file but named correctly (Test.ZIP) then I got a dialog asking me what to do with the file "Test.ZIP" of type application/x-zip-compressed, and the default application was given, Compressed Folder.
Attempting to open my zip file renamed "TESTz" (no extension) I got:
It would help if you added the following information:
The exact contents of the Netscape
Opening ____ box, including the location of the file,
AND
the complete URL of the Yahoo mail login page that causes it.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax)